Two modes of guidance for conducting a non-precision approach to an airport are known. The first mode is the angular mode. In this mode the aircraft must remain in a cone or pyramid whose vertex is the end of the runway in general and whose aperture is defined by an angle of precision. This first mode is also known by the acronym “FLS” for “FMS Landing System”. The second mode of approach is a geometric segment linear mode in which the aircraft must fly in a corridor centred on the landing runway in general and whose width depends on the precision necessary for landing.
In airports, non-precision approaches are carried out as a function of this linear mode. Thus if the pilot wishes to use an angular mode on approach, it is necessary to determine whether the use of this mode is compatible with the precision requirement of the approach in order to be compatible with the obstacles along the approach.
It is known in the prior art to leave it up to the crew to be responsible for using an angular guidance mode on an approach scheduled to be flown using a linear guidance mode. To aid their judgment the FMS represents the axis which will serve for the lateral guidance mode on the navigation screen and, when the aeroplane is provided with a vertical display, the axis which will serve for the vertical guidance mode. The divergences in trajectories are evaluated with the aid of deviation indicators.
When the angular guidance mode is used, the divergences along the approach are evaluated with the aid of indicators of angular deviation in place of the linear deviations. This prevents the crew from pinpointing the deviation in terms of linear divergence and this may lead the crew not to use the angular mode, although it is designed to improve the safety of the approach by its naturally convergent behaviour, comparable to that of ILS (acronym standing for Instrument Landing System).
It is also known to exclude a certain number of procedures of the angular guidance mode. This exclusion is carried out when the segments constituting the approach are not aligned with the last segment of the approach procedure. This is for example presented in the Honeywell patent U.S. Pat. No. 8,121,747B2. Whereas this scheme precludes the pilot from using an angular guidance mode on certain approaches that are manifestly incompatible therewith, it does not guarantee, however, that the approaches, which the patent permits, comply with the constraints of obstacles. Thus the crew remains in doubt as regards the relevance of using an angular guidance mode during an approach scheduled to use a linear guidance mode.
Also, approaches with a high requirement for precision, termed “RNP-AR” for Required Navigation Performance, are not eligible in the initially angular mode.